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1
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1answer
210 views

How many electrons are there in the universe?

Someone on io9 estimated there were about 10^80 electrons in the universe, but I want to ask the Stack Exchange physics community.
3
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3answers
100 views

Is it possible to calculate the radius of star?

Suppose I know the Luminosity $L$, temperature $T$ and Mass $M$ of star. Assuming the star is very heavy so that we can treat it to be radiation-dominated star. This would imply that pressure inside ...
-2
votes
1answer
111 views

Largest size of a low gravity, Earth-like planet with Volcanoes and Earthquakes?

Provided that the Solar System consists only of Sol, a planet, and the planet's moon.... How large can my planet be so that it has low enough gravity that teenage human girls trained for jumping, are ...
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2answers
189 views

Atoms in a piece of paper?

How many atoms are there in a common paper sheet? I mean, for example, in one of those we use for photocopies, numerically speaking, how many atoms in total? (Ten to the power of..?) And I'd like to ...
0
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0answers
69 views

Orbital elevators and the Coriolis effect

Consider an orbital elevator simply modelled as a large mass in geostationary orbit (altitude approx 36,000km) around the Earth, connected to a point on the equator via a strong, light, likely ...
1
vote
1answer
196 views

Continuity equation and pressure head in a tank

Consider a tank which is full of water. We connect a horozontal pipe at its bottom. When the pipe is open, water starts flowing through the pipe, then the velocity of water is proportional to the ...
1
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3answers
161 views

How do these factors affect the free fall of objects?

Take an example of two balls of different masses being dropped from about 250 meters from the ground. How do the following factors affect free fall: Air resistance. Absence of air resistance. ...
1
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1answer
27 views

Stair well descent

I am trying to understand the science behind lowering a person down a flight of stairs on a sled. My tensile strength only says it can hold 250 pounds but I have seen it lower people that weigh 300 ...
22
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5answers
8k views

Is it possible to shoot bullets in space or would the recoil of the gun be too strong?

I've read a few articles that say that astronauts have already brought guns in space and that shooting bullets in space is possible. But won't the recoil of the gun be too strong? Law of ...
6
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3answers
180 views

What would it take to cause lightning to jump between the Moon and the Earth?

This question comes from @Floris' speculation at the end of his excellent answer about what it would take to kill everyone on the Earth with electricity. Doing all this in 1/10th of a second ...
3
votes
1answer
102 views

Force and lions

Alright. So me and some other people right now need an answer to this. If you have 1 trillion lions traveling at 81 kilometers (top speed of a lion) towards the Sun in a pretty dense ball. (As dense a ...
29
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5answers
6k views

If the Earth is a good conductor of electricity, why don't people get electrocuted every time they touch the Earth?

Since the Earth is a good conductor of electricity, is it safe to assume that any charge that flows down to the Earth must be redistributed into the Earth in and along all directions? Does this also ...
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2answers
72 views

What should we do If we wanted to increase the angular velocity of a planet? [duplicate]

We could hit it with really fast objects, but could we manipulate the orbit of a large satellite to speed up its rotation? What would be the easiest way?
1
vote
1answer
25 views

Tangent and Normal accelerations position estimation

How can I derive a particle position given it's last known positions (x,y), velocities in it's components (vx, vy), tangential and centripetal (normal) accelerations? (this is the only available data) ...
1
vote
1answer
42 views

Tsunami effects of a nuclear explosion

Sea surface nuclear explosion arouses a tidal wave of water. How does this kind of wave compare to naturally occurring tsunamis? E.g. what would be the potential of a 100KT bomb 100km away the from ...
4
votes
3answers
295 views

What was the amount of energy released in the 9/11 terrorist attacks?

I read somewhere on the internet that the amount of energy released by the terrorist attacks on the twin towers on 9/11 should have been of the order of 100 tons of TNT. This number surprised me, ...
0
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2answers
44 views

How quickly does a gas reach a uniform temperature in an enclosure?

I'm just looking for ballpark figures and reasoning. Suppose a gas (air) at STP was introduced into a 10cm diameter sphere of infinite heat capacity and conductivity at one degree above the gas. How ...
1
vote
2answers
120 views

Would it be possible to make a space elevator only in the atmosphere?

I wonder if with current day technology we could make a "floating platform" which hangs from a satellite by a space elevator. This could allow a "launch process" involving floating a balloon to the ...
2
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3answers
111 views

How can Denver offer Hot Air Balloon rides?

So the air density in Denver is 1.0 kg/m^3. And heated air has the density of 0.95 kg/m^3. So adding the weight of the basket and balloon I don't understand how a hot air balloon could get off the ...
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5answers
6k views

Freefall into snow

In the movie Frozen, the following dialogue takes place: Anna: "It's a hundred-foot drop." Kristoff: "It's two hundred." Anna: "Okay, what if we fall?" Kristoff: "There's 20 feet of ...
2
votes
1answer
60 views

The fly and the sphere of iron

When I was a child, I read something like the following in a book whose name I have now forgotten (I tried to find the original quote on the web, but although there are some similar passages the ...
7
votes
1answer
704 views

How fast will sublimed dry ice mix with air?

I saw this photo and wondered: Will the CO2 stay mostly in a layer on the floor with the rest of the atmosphere resting on top, or will it quickly diffuse throughout the room? This lab is probably ...
2
votes
1answer
81 views

Why do flat sheets of paper twist more than paper cones as they fall?

If I cut a 10cm circle out of a piece of paper and drop it, it will twist as it falls, even if it's rigid. If I then cut out a one-quarter wedge of the circle and tape the remaining three-quarters ...
2
votes
1answer
84 views

How much potential fusion energy is in Earth's ocean?

Assuming the most efficient manner for extraction, and a ready supply of other necessary materials not mentioned herein, and given the current estimates about the volume of Earth's ocean, how much ...
0
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1answer
90 views

How much energy would it require to send Earth to Proxima Centauri?

Assuming 100% efficiency of energy usage, and given the current estimates about the mass of known objects in our solar system, including Earth itself, and assuming no other objects are affected by ...
3
votes
1answer
88 views

How powerful would a laser need to be to make a hole in snow?

How strong would a laser beam have to be (and what frequency would be optimal) for it to be able to make a hole in snow? Here I am looking for a laser beam that could make the hole after less than 2 ...
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3answers
141 views

How big would a solar sail need to be to be detected from the orbit of Pluto?

Suppose we made a solar sail out of a highly reflective material. How big would that solar sail have to be for the Hubble Space Telescope to detect it visually at the average distance of Pluto?
3
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1answer
135 views

What is the force required to move a raindrop?

I'm doing some research on how much force is required to move a typical raindrop (assuming that it is falling straight down) off to the side of X distance. This is for a school project on creating our ...
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6answers
4k views

Throwing an object around the Earth. Is it possible?

Some of you might be familiar with the cartoon called Astérix. In one of the episodes, Obelix (the really strong fat guy in the stylish striped pants) enters a spear throwing competition. He beats the ...
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6answers
2k views

Gay-Lussac's Law & the Patriots

This may be a question better suited for xkcd what if? section...but here goes: with all the hoopla around the patriots / colts game and allegations of cheating by deflating footballs, I've read/heard ...
1
vote
1answer
36 views

liquid trapped in wet sponge

If you soak a sponge in water, or some other liquid, and then let it hang then some liquid will flow down due to gravity but not all, some amount will remain trapped in the sponge by the capillary ...
1
vote
1answer
88 views

How size of objects lifted by the wind depends on the speed of the wind?

What are the sizes of the objects that could be picked up and flown by tornados, depending on the tornado's speed? To put it another way, assuming turbulent flow and object density much higher than ...
0
votes
2answers
178 views

A big cannon to match a ballistic missile? [closed]

During the last years of WW2 the Germans used ballistic missiles V-2 (with payload mass ~1,000 kg) to bombard London, from a distance about 300 km away. Suppose the British could respond by building a ...
3
votes
1answer
148 views

Estimate for minimum potential depth required for bound states in 3D attractive potential well

Consider a 3D spherically symmetric potential well, $$H = \frac{p^2}{2m} + V(r)$$ with $V(r) = - V_0$ for $r < a/2$ and $0$ else, for some $V_0 > 0$. Now, it is well known that $V_0$ needs to ...
0
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0answers
98 views

Algorithm for calculating lunar crescent visibility

Is there a simple algorithm for predicting lunar crescent visibility. Simple enough to do with a calculator at least. Those offered in Meeus' Astronomical Algorithms are a bit too involving to do by ...
0
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0answers
41 views

Can the half-life of rubidium 87 be theoretically estimated?

Can the Fermi golden rule be applied to give an approximation of this half-life?
3
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1answer
26 views

Can physicist estimate is it safe to walk/drive on ice? [closed]

I'm curious about the following: Suppose that I know the the weather forecasts of a given place. How can I compute from that data whether or not it is secure to walk or drive by a car to an island.
3
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1answer
73 views

Mammal Population Estimates as a Fermi problem [closed]

Can We estimate the order of magnitude of number of mammals on earth? Can this be treated as Fermi problem or not?
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2answers
147 views

Simple estimation of the critical temperature of water

I'm trying to develop fermi estimation skills and I came up with a question for which I don't even know where to start from. Here goes: Is it possible to estimate the critical temperature (say in ...
1
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2answers
215 views

7 Billion people jumping at once

What would happen if every person in the world gathered at a minimum possible area and jumped all at once? Would that have any effect on Earth?
0
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1answer
94 views

How to calculate the Impact of a meteor

I am trying to add a dose of realism to a story I'm writing: imagine an invulnerable superman-like being that won't disintegrate at any point, weighing in at 349lbs, travelling at 86,000mph through ...
2
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1answer
68 views

Is there a Fermi estimations toolbox?

I am a theoretical physics student and am a little ashamed at my inability to estimate any measurable quantity. I would like to develop my skills at Fermi estimations. Although it is hopeless to ...
2
votes
3answers
388 views

Power to suspend a mass in air

How much power is needed to suspend a mass in the air? The four parts below deal with the same problem. I post them all here, should there be some subtle things I didn't see. The textbook problem A ...
2
votes
3answers
160 views

Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
2
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0answers
31 views

Quantum Efficiency Estimation

Might there be a way to do a rough estimate of the quantum efficiency of a photo-detector like a CCD or CMOS sensor based only on a picture taken with it? I've read papers and guides (like this one: ...
4
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3answers
248 views

What was the density of the universe when it was only the size of our solar system?

What was the density of the universe when it was only the size of our solar system? Did it approach neutron star density? Is it physically correct to even ask such a question?
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2answers
1k views

Temperature on the surface of the sun calculated with the Stefan-Boltzmann-rule

In a German Wikipedia page, the following calculation for the temperature on the surface of the Sun is made: $\sigma=5.67*10^{-8}\frac{W}{m^2K^4}$ (Stefan-Boltzmann constant) $S = 1367\frac{W}{m^2}$ ...
0
votes
1answer
329 views

How thick (in molecules) would a piece of paper be if it was folded to the moon? [closed]

Apparently you can fold a piece of paper 42 times and reach the moon! 42 is the answer the everything after all! Oh, Douglas Adams, you clever son of a bitch. The reasoning behind this is that as you ...
4
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2answers
559 views

Ten-ping bowling: Can a ping pong ball knock over a bowling pin?

In this video we see a rather unorthodox bowling technique on display. The gentleman appears to knock over all of the pins using only a ping pong ball. It's a fake, of course: you could never knock ...
5
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1answer
101 views

Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?

I can calculate that if you want to, for example, desalinate water, you will have to pay a free energy cost of $k_B T$ for each ion you remove. In other words, removing an ion from a volume of water ...